(a) Field of the Invention
The present invention relates to a method for correcting an optical proximity effect and, more particularly, to an optical proximity effect correction method for correcting the shape of a corner portion of a mask interconnect pattern to be transferred onto a photoresist film by a photolithographic technique.
(b) Description of the Related Art
Along with reduction in design rule for semiconductor devices, patterns for forming the semiconductor devices, such as patterns for gate electrodes, interconnect layers and contact holes, have been more and more reduced in the dimensions thereof Accordingly, mask patterns formed on an exposure mask, which are used for patterning the gate electrodes, interconnect layers and contact holes by photolithography, have also been more and more reduced in the dimensions thereof.
In an optical exposure in photolithography, When a mask pattern which has been reduced in dimensions to a degree close to the critical resolution of an exposure system is exposed and transferred onto a photoresist film, or the like, adjacent light beams optically interfere with each other during forming finely patterned regions which are in close proximity to each other. As a result, the exposed image is distorted. Thus, it is not possible to precisely transfer the mask pattern of the exposure mask. Generally, this is called an xe2x80x9coptical proximity effectxe2x80x9d.
If an optical proximity effect occurs in the exposure transfer process, the design pattern cannot be precisely transferred onto the photoresist film by using the mask pattern, whereby it is not possible to obtain device characteristics as desired in the design stage of the semiconductor device.
In view of the above, the mask pattern is generally subjected to a correction for compensating for the optical proximity effect in order to suppress the optical proximity effect and thus to precisely transfer the mask pattern of the exposure mask onto the photoresist film.
Now, referring to some of attached drawings, the phenomenon of an optical proximity effect and a correction method therefor will be briefly described. FIGS. 1A and 1B are schematic diagrams respectively illustrating a mask interconnect pattern of an exposure mask, and an actual interconnect pattern which has been distorted by the optical proximity effect after exposing and transferring the mask interconnect pattern onto a photoresist film. FIGS. 2A and 2B are schematic diagrams respectively illustrating a mask interconnect pattern which has been corrected so as to compensate for the optical proximity effect, and an actual interconnect pattern obtained therefrom.
A desired design interconnect pattern, i.e., a mask interconnect pattern 11 of an exposure mask which is in a predetermined relationship with (e.g., in a constant dimensional proportion to) the desired design interconnect pattern, is an L-shaped pattern having a straight portion 12 and a corner portion 13 which is bent in a direction at 90xc2x0 with respect to the straight portion 12, as illustrated in FIG. 1A, and the pattern is arranged so that the corner portion 13 of the interconnect pattern 11 is positioned above a contact hole 14. The outer corner located at the outer periphery of the corner portion 13 has a bend angle xcex81 of 90xc2x0 as measured from within the interconnect pattern 11, and is the inner corner located at the inner periphery of the corner portion 13 has a bend angle xcex82 of 270xc2x0.
When the mask interconnect pattern 11 as illustrated in FIG. 1A is exposed and transferred onto the photoresist film, the interconnect pattern 16 obtained by the transfer process has round corners on the outer and inner peripheries of each of the corner portions 13. That is, each corner of the corner portion 13 is rounded by the optical proximity effect, as illustrated in FIG. 1B, thereby causing a deviation with respect to the contact hole 14. As a result, the contact area between the interconnect pattern 16 and the contact hole pattern 14 is reduced, whereby the contact resistance may increase and even a connection failure may occur therebetween.
In view of the above, a mask pattern having such a correcting pattern as to compensate for the optical proximity effect is used in the conventional technique in order to suppress the optical proximity effect.
For example, in Japanese Patent Laid-Open Publication No. Sho-10-229124, in order to suppress the above-described optical proximity effect, an additional correction pattern 17 or an xe2x80x9cLxe2x80x9d-shaped additional pattern is added to the outer corner of the corner portion 13 of the mask interconnect pattern 11, and a cutout correction pattern 18 or a xe2x80x9cLxe2x80x9d-shaped cutout pattern is provided on the inner corner of the corner portion 13.
By using such an exposure mask having a mask interconnect pattern for which the optical proximity effect has been corrected, it is possible to obtain a transferred interconnect pattern 19 close to the desired mask interconnect pattern 11, as illustrated in FIG. 2B.
The respective dimensions of the additional correction pattern and the cutout correction pattern which are provided in the mask pattern of the exposure mask in order to provide the correction of the optical proximity effect are determined by experiments, etc., for each of desired L-shaped interconnect patterns.
For example, as for the cutout correction pattern 20 provided on each inner corner of the corner portion 13 shown in FIG. 3, the cutout correction pattern 20 is generally an xe2x80x9cLxe2x80x9d-shaped pattern having two legs extending in two directions along the inner periphery of the corner portion 13. The two legs of the xe2x80x9cLxe2x80x9d-shaped pattern 20 meet each other at a right angle, and have a length of L and a width of W, as illustrated in FIG. 3.
The dimensions L and W of the cutout correction pattern are predetermined by experiments, etc., for each of the process conditions and the widths of the interconnects to be used, so as to eliminate the optical proximity effect as much as possible by providing a suitable correction. These dimensions are recited in a so-called xe2x80x9coptical proximity effect correction rulexe2x80x9d.
An interconnect pattern provided in a semiconductor device is basically formed by an L-shaped pattern which includes a straight portion and a corner portion. An ordinary interconnect pattern has generally no outer corner or inner corner having a bend angle other than 90xc2x0 and 270xc2x0. Thus, for the purpose of designing and fabricating semiconductor devices, it is practically correct that any outer corner formed at the outer periphery of the corner portion has a bend angle of 90xc2x0 and an inner corner formed at the inner periphery of the corner portion has a bend angle of 270xc2x0.
Therefore, in the conventional technique, when providing an optical proximity effect correction for a mask interconnect pattern, a cutout correction pattern having predetermined dimensions was equally applied to the inner corner portion of the corner portion of L-shaped interconnect pattern with reference to the predefined optical proximity effect correction rule.
For example, when designing a mask interconnect pattern for representing a simulation design interconnect pattern 22 as illustrated in FIG. 4, the design interconnect pattern 22 is scanned along the layout of the design interconnect pattern. In this process, each time one of corner portions 23 to 31 is extracted, a cutout correction pattern and an additional correction pattern based on to the optical proximity effect correction rule is automatically and equally provided to the inner corner and the outer corner of the one of the extracted corner portions 23 to 31.
In FIG. 4, numeral 32 denotes a straight portion between adjacent corner portions 24 and 25, numeral 33 denotes a straight portion between adjacent corner portions 27 and 28, numeral 34 denotes a straight portion between adjacent corner portions 30 and 31, and numeral 35 denotes a diffused region. Each of the straight portion 32 between the corner portion 24 and the corner portion 25, the straight portion between the corner portion 26 and the corner portion 27, and the straight portion between the corner portion 28 and the corner portion 29 implements a gate electrode.
In the above-described conventional method in which an interconnect pattern is handled as a combination of L-shaped patterns or, in other words, in the optical proximity effect correction method in which an interconnect pattern is divided into L-shaped patterns, there are problems as described below. FIG. 5 shows a schematic interconnect pattern diagram illustrating the problems in the conventional optical proximity effect correction method.
A mask interconnect pattern has a xe2x80x9c]xe2x80x9d-shaped pattern. The xe2x80x9c]xe2x80x9d-shaped pattern as used in this text is also called a double-L pattern which includes two corner portions 37 and 38 opposing each other so that the inner corners (or the corners having a bend angle of 270xc2x0) of both the corner portions 37 and 38 oppose each other, and that the corner portions 37 and 38 forming the xe2x80x9c]xe2x80x9d-shaped pattern are disposed in close proximity to each other, with a gap therebetween below a specified threshold distance.
If a cutout correction pattern 39 is equally provided on the inner corner of each of the corner portions 37 and 38 of the double-L pattern 36 based on the optical proximity effect correction rule, the resultant interconnect pattern 40 of the mask obtained by using the cutout correction patterns 39 has a small distance xe2x80x9cdxe2x80x9d between opposing ends of the legs of adjacent L-shaped correction patterns, as illustrated in FIG. 5. In this case, the straight portion 41xe2x80x2 of the exposed interconnect pattern indicated by dotted lines between the corner portions 37 and 38 has a significantly smaller width compared to the design width, thereby resulting in a locally increased resistance in the interconnect pattern 36.
Particularly, if the straight portion 41xe2x80x2 implements a gate electrode 41 overlying a diffused region 42, as illustrated in FIG. 5, the gate electrode 41 has a length of L2 which is smaller in length than the design gate length L1. In the current semiconductor devices, the gate length has been more and more reduced, and the short channel effect of a transistor is sensitive to a slight difference in the gate length L. Thus, this may cause a significant problem that the transistor characteristics may significantly deviate from the design characteristics, whereby it is not possible to obtain a precise circuit operation as desired.
In the above-described example, if the cutout correction pattern 39 is not provided in the mask interconnect pattern 36 in the exposure mask, as illustrated in FIG. 6, the gate length of the straight portion 41xe2x80x2 i.e., the gate electrode 41, has the desired gate length L1. However, the end portion 43 of the gate electrode 41 has a larger width and thus the gate electrode has a larger length, thereby again deteriorating the transistor characteristics.
In view of the above, it is an object of the present invention to provide a method for correcting an optical proximity effect for correcting a mask pattern so that when forming an interconnect pattern having a xe2x80x9c]xe2x80x9d-shaped pattern or double-L pattern, the line width of the straight portion of an interconnect between the corner portions has a suitable width substantially equal to the design width.
The present invention provides a method for correcting an optical proximity effect in a photolithographic process for pattering an interconnect pattern by using a mask pattern having a mask interconnect pattern, the method comprising the steps of extracting corner portions in the mask interconnect pattern, providing a default cutout correction pattern having an xe2x80x9cLxe2x80x9d-shape on an inner corner of each of the extracted corner portions, calculating a first distance between opposing ends of legs of adjacent two of the default cutout correction patterns, comparing the first distance against a threshold distance, and modifying the default cutout correction patterns to obtain modified cutout pattern if the first distance is larger than the threshold distance so that a distance between opposing ends of legs of the modified cutout patterns is substantially equal to the threshold distance.
In accordance with the method of the present invention, the straight portion between the corner portions disposed in a close proximity is prevented from being made smaller than the design width due to the modified cutout correction patterns disposed on the inner corners of both the corner portions.
The above and other objects, features and advantages of the present invention will be more apparent from the following description, referring to the accompanying drawings.